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Carbon brushes for
fractional horsepower motors
Radio interference suppression in commutator motors
As a result of the constantly rising
number of radio-receiving devices,
radio interference suppression is of
great importance for electrical motors
in household and industry. Since
these devices are primarily equipped
with commutator motors, the resulting oscillations cause high frequency
harmonic waves, which represent an
interfering signal. This interfering
signal superimposes and disturbs
the wanted signal of radio traffic,
radio broadcasting and television.
Consequently legally binding limits
were specified. All devices, which generate electromagnetic oscillations within the range of 10 kHz to 300 MHz,
need to be approved according to the
high frequency law.
The regulations dealing with radio
interference comprise broadband
frequency spectra, which result from
rapidly occurring electrical processes
with a repetition rate of at the furthest 10 kHz in connection with mechanical contacts, semiconductors
and commutator motors. The resulting radio interferences are frequency-dependent transmitted to receiver
devices both via cable and wireless.
The radio interference of the devices
powered by electric motors is presently measured in principle in a frequency range of 0.15 to 300 MHz,
whereby in Western Europe the values
for radio interference voltage and interference output must be kept within the limiting values determined by
CISPR (Comitée International Spécial
des Perturbations Radioélectriques).
The IEC international special committee for radio interference prepares the CISPR regulations.
These regulations correspond to
the following EN-regulation DIN EN
55014-1.
Because of the wavelength, the radio
interference voltage is transmitted in
the frequency range of 0.15 – 30 MHz
mainly via cable (connecting lines).
Therefore the radio interference voltage is measured within this range.
Within the range of 30 – 300 MHz the
radio interference is caused almost
exclusively by interfering radiation.
The dimensions of the devices, into
which the motors are installed, are
an important factor. Two types of
measuring methods are used here:
on the one hand the actual field intensity measurement by means of a
suitable antenna, on the other hand
the “clamp measurement” with absorption clamp and radio interference output measurement receivers,
a method developed in accordance
with the CISPR. This latter method
can be performed with relatively
small expenditure in regards to instrumentation. The testing laboratory uses the antenna measurement in
arbitration cases. Details regarding
the methods and limiting values
according to the G, N and K levels of
radio interference can be found in
the aforementioned regulation.
The devices are basically differentiated according to their intended purpose:
Schunk Kohlenstofftechnik
a) Household appliances
powered by electric motors
b) Hand-held electric power tools
Since the introduction of the artificial
mains network with 50 Ω // 50 µH +
5 Ω a special modified limiting curve
for the interference level N applies to
household appliances powered by
electric motors.
For hand-held electric power tools,
three limiting curves for the interference level N still apply, which are distinguished depending on the rated
power consumption of the devices.
The exact limiting curve according to
the interference level N applies to
the interference suppression of
devices with up to 700 W power consumption. The limiting value is 4 dB
higher for devices with 700 – 1000 W
power consumption. For devices with
a power consumption of more than
1000 W a limiting value that is 10 dB
higher can be used as a basis.
The radio interference of commutator motors is caused by mechanical
and electrical influences, which often
can be reduced by relatively simple
means. Therefore it is at a later date
possible to reduce expensive suppression material.
Electronically speaking it is important to keep the energy content of
the interference source to a minimum, which means that the internal
resistance of the interference source
must be large.
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Radio interference suppression in commutator motors
Brush sparking occurs in commutator motors as a result of the commutation process and possible mechanical malfunctions. This leads to high
spikes above the segment voltage
with a steep edge. It can be proven
with Fourier analysis that such a
spike can be divided into harmonious sinus oscillations, which reach
into the highest of frequencies. The
shorter and steeper such a spike is, the
further the frequency spectrum extends into the high frequency range.
Phase controls with thyristors or
triacs as well as rectification with
silicon semiconductors, which are
nowadays installed in an increased
extent in household appliances and
hand-held electric power tools, are
an additional source of interference,
mainly in the long wave and medium
wave range.
The resulting high-frequency oscillations are transmitted as interference
voltage via the line cord and as interference field strength and/or interference output through radiation in
free space.
When designing the motor, the number of segments and the number of
rotor slots should be chosen as high
as possible in order to reduce the
segment voltage (for 230 V motors
e. g. 24 segments and more, if possible). The exciting coils should be
arranged as symmetrical as possible
and one-sided field tapping for different speeds should be avoided.
To ensure a smooth mechanical carbon brush run and thus reduce radio
interference, the following points
should be considered:
The residual imbalance of the motor
should be as small as possible, e. g.
0.5 – 1.5 gmm, and the axial freedom
of motion of the rotor should be
restricted as much as possible.
Every effort should be made to avoid
any eccentricity of the commutator
after the final manufacturing operation, otherwise mechanical excitations will occur periodically on the
carbon brushes.
A limiting value of 8 µm should not
be exceeded for motors with speed
values up to 25000 rpm. For motors
with a speed range of 25000 –
35000 rpm a limiting value of < 6 µm
applies and a limiting value of < 4 µm
applies for motors with a speed values of > 35000 rpm.
The out-of-roundness of the commutator must be kept to a minimum as
well. The same limiting values as
shown with eccentricity apply for
out-of-roundness when distributed
over a larger commutator area. Outof-roundness caused by protruding
segments should not exceed a value
of 1 – 2 µm, depending on the size of
the commutator, otherwise an increase
in commutator sparking is to be
expected due to the mechanical contact separation.
A commutator surface roughness of
RZ = 6 – 12 µm and/or Ra = 1 – 1.6 µm is
required to improve the running in
of the carbon brush and/or to increase its service life.
If the surface roughness is too low,
the coefficient of friction will be too
high which results in an unsteady
brush run.
It should be further mentioned that
the clearance between carbon brush
and the brush holder affects the carbon brush running behaviour and
especially the radio interference.
2
Tolerance guidelines according to the
DIN IEC should therefore be observed
unconditionally. Please see details to
this in brochure 13.34.
The carbon brush material and the
carbon brush contact pressure must
of course be optimally coordinated
with the motor. By using a carbon
brush material with an impedance as
high as possible, the short-circuit
current can be limited and the internal resistance of the interference
source can be increased. Additionally
it is possible to lower the radio interference during the entire carbon
brush lifetime through purposeful
impregnations, e. g. with symbol F7,
F10, F12, F13, F20, F25, F101, F131 and
ZP, and through improvement of the
mechanical running properties and
therefore better electrical behaviour.
Furthermore the range of the measured values is reduced. The impregnations stated here remain stable
and effective in their concentration
and effectiveness up to the end of
the carbon brush service life, coordinated with defined maximum operating temperatures in the devices.
In motors that run unsteadily it is
sometimes of advantage for the interference suppression and the lifetime
to angle the brush top instead of having the standard design. Such a
measure provides a pressure component in the direction of the holder
wall in addition to the pressure component perpendicular on the carbon
brush. This brings the carbon brush
into a defined position and is good
for vibration damping.
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Schunk Kohlenstofftechnik GmbH
Hoffmann & Co., Elektrokohle AG
Rodheimer Strasse 59
35452 Heuchelheim
Germany
Au 62
4822 Bad Goisern
Austria
Phone: +49 (0) 641 608-0
Fax:
+49 (0) 641 608-1494
Phone: +43 6135 400-0
Fax:
+43 6135 400-10
www.schunk-group.com
infobox@schunk-group.com
www.hoffmann.at
office@hoffmann.at
13.36e/1000/2008